Error Log: Deprecated, headers sent

Hi there,

thanks for the great plugin! Works great most of the time but every now and then I get this error when I click on an PDF link:

Deprecated: Methods with the same name as their class will not be constructors in a future version of PHP; mPDF has a deprecated constructor in URL/wp-content/plugins/dk-pdf/includes/mpdf60/mpdf.php on line 89 Deprecated: Methods with the same name as their class will not be constructors in a future version of PHP;
grad has a deprecated constructor in URL/wp-content/plugins/dk-pdf/includes/mpdf60/classes/grad.php on line 3 Deprecated: Methods with the same name as their class will not be constructors in a future version of PHP;
mpdfform has a deprecated constructor in URL/wp-content/plugins/dk-pdf/includes/mpdf60/classes/mpdfform.php on line 3 Deprecated: Methods with the same name as their class will not be constructors in a future version of PHP;
cssmgr has a deprecated constructor in URL/wp-content/plugins/dk-pdf/includes/mpdf60/classes/cssmgr.php on line 3 Deprecated: Methods with the same name as their class will not be constructors in a future version of PHP;
otl has a deprecated constructor in URL/wp-content/plugins/dk-pdf/includes/mpdf60/classes/otl.php on line 12 Deprecated: Methods with the same name as their class will not be constructors in a future version of PHP;
TTFontFile has a deprecated constructor in URL/wp-content/plugins/dk-pdf/includes/mpdf60/classes/ttfontsuni.php on line 51
Warning: Cannot modify header information – headers already sent by (output started at URL/wp-content/plugins/dk-pdf/includes/mpdf60/mpdf.php:89) in URL/wp-content/plugins/dk-pdf/includes/mpdf60/mpdf.php on line 8297 Warning: Cannot modify header information – headers already sent by (output started at URL/wp-content/plugins/dk-pdf/includes/mpdf60/mpdf.php:89) in URL/wp-content/plugins/dk-pdf/includes/mpdf60/mpdf.php on line 1706 mPDF error: Some data has already been output to browser, can’t send PDF file